When the algebraic difference is negative, it is called interference, and the hole size is less than the shaft size. This kind of interference (including the minimum interference is zero) is called interference fit. Because of the interference, there is no relative movement between the shaft and the hole, but it can bear certain torque and axial force. For example, if the size of the hole is 19.98mm, and the size of the shaft is 20.02mm, then the interference of the two is 19.98-20.02 = -0.04mm. At this time, the shaft and the hole are closely combined without any relative movement.
According to the magnitude of interference, interference can be divided into three types: mild interference, moderate interference and severe interference. Generally, the interference between 0 ~ 0.05mm is called mild interference, the interference between 0.06 ~ 0.10mm is called moderate interference, and the interference between 0.11 ~ 0.15mm is called severe interference. According to different matching properties, parts shape and process requirements, the assembly methods of interference fit are also different. Generally speaking, there are three kinds of assembly methods: press fit, hot fit and cold fit. In the repair, two methods are commonly used: press fit and hot mount, while in automobile manufacturers, cold assembly is more often used. For example, when installing bearings, the bearings are usually put on the shaft after being heated, while in the main engine plant, the shafts are generally cooled, and the normal temperature bearings are put on the shafts after the shafts are shrunk.
Interference fit is also very common in automobiles. For example, between the flywheel ring gear and the flywheel, the gear ring is usually heated and then put on the flywheel; there is also a crankshaft flange, which is set on the front section of the crankshaft, and its interference is relatively large, which needs to be installed in place by hot mounting and pressing. There is also the matching between the gear wheel and the wheel hub, which is also a very large interference fit, as well as various pins and keys. These interference fit parts are the most troublesome parts in the disassembly, and often need destructive disassembly.
Transition fit means that there may be clearance fit or interference fit when the hole and shaft are assembled. At this time, the tolerance zone of the hole overlaps with that of the shaft. It is mainly used in the connection parts with high requirements for neutral and coaxiality, which is convenient for disassembly and accurate positioning. The most common is the matching between the inner diameter of rolling bearing and the shaft. The interference is generally less than 0.02mm at room temperature. If the bearing is heated slightly or the shaft is cooled, the two will become zero clearance or positive clearance, which can be easily disassembled. In addition, the fit between the piston pin and the piston is also a transitional fit. At normal temperature, there is a certain amount of interference between the two. When the engine works normally, the piston expands when it is heated, and there is a certain gap between the two, so that the piston pin can rotate on the piston, which is conducive to reducing wear. This connection method is called full floating piston pin.